निम्नलिखित में से कौन-सा घोष वर्ण है?

In Hindi phonology, consonants are classified into Aghosh (Voiceless) and Ghosh/Saghosh (Voiced).
* Aghosh (अघोष): The 1st and 2nd letters of each consonant class (वर्ग) + श, ष, स.
* Ghosh/Saghosh (घोष/सघोष): The 3rd, 4th, and 5th letters of each consonant class + य, र, ल, व, ह + all vowels.

Let's check the options:

* 'च' is the 1st letter of च-वर्ग (Aghosh).
* 'ख' is the 2nd letter of क-वर्ग (Aghosh).
* 'ठ' is the 2nd letter of ट-वर्ग (Aghosh).
* 'म' is the 5th letter of प-वर्ग (Ghosh).

Therefore, म is a Ghosh वर्ण.